The present invention relates to a tire with a high transverse curvature coefficient, in particular suitable for fitting to motorcycles.
The invention relates in particular to tires for two-wheeled vehicles, in which the transverse curvature coefficient, which in any case has a value of not less than 0.15, is greater in the front tire than that of the corresponding rear tire.
Even more particularly the invention relates to tires which have a carcass with a radial structure and preferably have a cross-sectional ratio (H/C)&lt;80% and/or are mounted on a mounting rim with a width&gt;60% than the nominal chord of the tire.
Generally tires for motorcycles comprise a torus-shaped carcass with a central peripheral crown and two sidewalls terminating in a pair of beads for securing the tire to a corresponding mounting rim, a tread extending coaxially around the peripheral crown and a belt structure arranged between the carcass and the tread.
The carcass in turn comprises at least one rubberized fabric ply provided with reinforcing cords which moreover are of the textile type.
The textile cords are identified by a numerical abbreviation which represents the count of the fibre used and the number of strands which combine to form the cord. "Strand" is understood as being a bundle of filaments or single filaments which are twisted together; "count" is understood as being the weight in grammes of a length of 10,000 meters of fibre, expressed in dtex units.
As is known, a two-wheeled vehicle, when travelling along a curved trajectory, inclines on the sidewall inside the bend at an angle, called the camber angle, the value of which can reach a value as high as 65.degree. with respect to the plane perpendicular to the ground; this inclination produces on the part of the tires a (camber) thrust which opposes the centrifugal force acting on the vehicle.
It is also known that, with an increase in speed and a decrease in the radius of curvature of the trajectory, the inclination of the vehicle alone is no longer sufficient to compensate for the effect of the centrifugal force, ensuring the stability of travel of the vehicle itself: an increase in the thrust exerted by the tires is required, and this increase is obtained by varying the position of the vehicle with a manoeuvre, performed by the rider via the handlebars, usually known to experts as "push steering", i.e. inclining the rolling plane of the front tire with respect to the direction tangential to the curved trajectory at an angle, called "slip angle", directed in the opposite direction to the curvature of the trajectory.
This therefore produces a total thrust which is the sum of a camber thrust resulting from the inclination of the equatorial plane of the tire with respect to the vertical and a slip thrust caused by the angular variation in the rolling plane of the front wheel.
The value to be assigned to the slip angle depends on the structural and performance characteristics of the front tire, i.e. on the relationship which the tire is able to express between the value of the slip angle and that of the slip thrust, in combination with its camber thrust and that exerted by the rear tire.
In particular, in order to control steering of a motorcycle in the aforementioned conditions, it is preferred to modify the structural characteristics of the tires represented by the structure and the associated belt reinforcing elements.
A known tire for motorcycles comprises a carcass structure formed by a pair of rubberized fabric plies reinforced with cords which are symmetrically inclined with respect to the equatorial plane of the tire, usually known as a cross-ply carcass, and if necessary a belt structure which is also formed with pairs of rubberized fabric strips, with cords which are arranged angled both with respect to the equatorial plane of the tire and the circumferential direction thereof.
The abovementioned tire structure is able to generate considerable camber thrusts, but gives rise to problems relating to comfort, stability and road-holding power of the vehicle as well as fatigue on the part of the rider, caused by the excessive rigidity of these tires.
These drawbacks have been partly overcome by a tire which has appeared on the market more recently, comprising a radial carcass combined with a belt structure consisting of textile or metal cords: in particular the tire for use on the rear wheel is provided with a belt structure comprising, and sometimes in an exclusive manner, a wound arrangement of cords which are preferably metallic and oriented in the circumferential direction, while the tire for use on the front wheel retains the belt structure with radially superimposed strips of inclined cords, indicated above.
This pair of tires has undoubtedly improved the situation from the point of view of riding comfort and stability, particularly during travel on straight roads.
The improvement in the behaviour along straight roads has, however, given rise to a new problem, consisting in the fact that the radial carcass of the tire, in combination with a belt of cords arranged circumferentially (at 0.degree.), during travel around bends is not always able to produce a camber thrust which is suited to requirements, also taking into account the increasingly higher performances of vehicles.
More precisely, the rear tire provides a thrust which is quantitatively less than and qualitatively different from--i.e. is of a linear rather than a curvilinear nature--that of the front tire and this fact makes steering of the vehicle around bends even more critical.
Other tires for motorcycles, together with their structural characteristics, are for example described in the European Patent Applications EP 756,949 and EP 808,730 in the name of the Applicants, to which reference should be made for further more precise details.
Patent Application EP 756,949 describes a tire, in particular for use on the front wheels, which comprises a radial twin-ply carcass containing nylon cords with a count of 940/2 dtex and two radially superimposed belt strips provided with nylon cords with a count of 940/2 dtex arranged symmetrically intersecting with one another in the two strips and with respect to the equatorial plane of the tire. The belt structure further comprises one radially external layer of steel cords, with the constructional characteristics 3.times.4.times.0.20 HE HT, where the cords are distributed with a density which continuously increases from the centre towards the ends of the belt.
Basically, according to the technical solution described in this patent application, it is possible to obtain a belt which at the same time is flexible in the centre, in order to absorb and dampen the vibrations due to the roughness of the ground, and rigid laterally, so as to generate notable slip thrusts, by providing a central portion which is mainly provided with cords at zero degrees, in combination with a low density or total absence of reinforcing elements oriented in the transverse direction, and two side portions provided both with cords at zero degrees and with reinforcements oriented in an intersecting direction with respect to the equatorial plane.
These tires, compared to tires which have a radial twin-ply carcass made of nylon 940/2 dtex and a single pair of belt strips with cords intersecting with one another in the two strips, have provided better results with regard to the capacity for absorption of the roughness of the road surface (contact feeling), the behaviour of the vehicle (ease of handling) and the stopping distance (braking capacity).
Patent Application EP 808,730 describes a pair of tires for a motorcycle, i.e. a front tire and a rear tire.
More specifically, the front tire comprises a radial twin-ply carcass with textile cords made of a material having a higher count than that of the tire according to the preceding application, i.e. rayon 1220/2 dtex instead of nylon 940/2 dtex.
The belt consists of a radially external layer of metal cords, which are oriented circumferentially and have an axial variable density, and a radially internal layer consisting of a mixture reinforced with aramide paste, oriented in the circumferential direction.
In turn, the rear tire comprises a radial twinply carcass comprising cords made of nylon with a count of 1400/2 dtex and a belt layer with metal cords oriented circumferentially and distributed with a constant density.
These pairs of tires were compared with pairs of traditional tires where the front tires had a nylon radial twin-ply carcass and a belt structure consisting of a pair of rubberized fabric strips with aramide cords which are symmetrically inclined with respect to one another in the two strips, while the rear tires had a nylon twin-ply carcass and belt with circumferential cords made of aramide.
The outcome of the comparison between the pair of tires according to this latter patent application and those according to the prior state of the art also highlighted improvements, in particular as regards the capacity for absorption of the roughness of the road surface, the road-holding power and the stopping distance.
In particular, according to this solution, during travel around bends, the front tire has a performance which is qualitatively identical to that of the rear tire, i.e. is of the linear type: as a result, the vehicle acquires a neutral behaviour in terms of steering, which solves the problem arising with the pairs of tires according to the known art, essentially consisting in over-steering of the vehicle, due to the inability of the rear tire to oppose the centrifugal force in the presence of a front tire which is able to develop a camber thrust sufficient to counterbalance the centrifugal force, with a consequent tendency of the motor vehicle to skid towards the outside of the bend.
Therefore the current state of the art indicates the direction for research into further improvements in the use of belts comprising at least one layer of metal cords arranged at a substantially zero angle with respect to the equatorial plane and distributed with an axially variable density, in combination with carcasses made of rubberized fabric comprising textile reinforcing cords with a count which is increasingly higher than that of nylon fabric having cords with a count of 940/2 dtex, and hence fabrics which are stronger, even though of increasing weight.
All of this having been stated, the current demands of motorcycle manufacturers are pushing research towards further improvements in tires, in order to achieve a greater travelling stability of the vehicle, better grip of the tire on the ground and in particular better handling of the vehicle, obviously without adversely affecting all the other performance characteristics during riding.
In particular, there is a growing demand for easier handling in order to improve the performance of the vehicle on mixed travel surfaces, consisting of bends in either direction alternating with straight sections, and in particular for tires which are intended for competitive use, in a desperate attempt to reduce track lapping times in tests and competitions.
This demand may be satisfied by reducing the deformability of the tire carcass and increasing its rigidity, so as to allow more rapid transmission of the forces between the hub of the wheel and the ground, through the carcass, but this results in a reduction in comfort along straight sections, which, as mentioned, must be avoided.
Following the teachings of the state of the art, the Applicants have thought to use the carcasses formed by fabrics with high-count cords which had produced the best results, attempting to safeguard and possibly further increase the quality of the performance of the tire with further modifications of other structural parts.
In order to improve the handling, it was initially thought to continue using the carcasses with high-count rayon cords, using a greater height for the backfolds in the sidewalls and a filling with a rubber mixture having a greater hardness than that traditionally used; however, the greater rigidity conferred on the carcass, although producing a certain improvement with regard to the handling, gave rise to vibrations (or "shimmy") during travel with a consequent loss of stability of the vehicle, loss of comfort and shorter life of the tire owing to irregular wear of the tread.
Research, while keeping the rayon carcass unchanged, then concentrated on finding a mixture for the tread which would have a high modulus of elasticity so as to contribute to a greater rigidity of the tire at the same time as better grip on the ground. These attempts also did not produce satisfactory results.
During the carrying-out of the various comparative tests it was found that a tire made at random using a carcass fabric available in the warehouse, containing textile cords with a count of 550/2 dtex, produced a significant improvement in performance in terms of the time for completing one lap of the track, compared to a tire which hitherto was considered to offer the best performance and which had a carcass fabric provided with higher-count rayon cords, while maintaining the same levels of comfort and response.
The result obtained was entirely unexpected in that it was contrary to what the improvements of the state of the art had taught and what was logically predictable.
The subsequent analysis, in view of the positive nature of the result, involved detailed identification of the structural parts of the fabric and their characteristics.
The fabric achieving this improvement was identified as being a type already described in the pending patent application 97EP-830499.6 in the name of the same Applicants relating to motor vehicle tires provided with a carcass having low-count cords, specifically made of PEN.
The fabric was aimed at reducing the weight of motor vehicle tires.
The fact that this fabric with low-count cords had not been identified initially as a possible improving factor in a motorcycle tire can be explained by the different behaviour, around bends, of the two types of tire (for cars and for motorcycles, respectively), the aforementioned fabric not having demonstrated any particularly interesting characteristics in car tires, apart from its said lightness.
In fact, a motor vehicle tire around bends remains substantially in contact with the ground along its entire axial extension and its performance characteristics depend mainly on the characteristics of the belt structure, as demonstrated by the "bean-shaped" deformation of the impression area of the tire, which is entirely absent in the case of motorcycle tires.
In a different manner, the motorcycle follows a curvilinear trajectory, inclining the equatorial plane of the tires at angle values which are fairly high with respect to the vertical line to the ground: the impression area of the tire is not deformed in a "bean-like" manner, but is displaced axially from the equatorial plane towards the end of the tread and vice versa when entering a bend and leaving a bend.
This displacement of the impression area in combination with the inclination of the rolling plane of the tire requires an accentuated curvature of the tread in order to prevent, when travelling around a bend, the sidewall of the tire from coming into contact with the road.
For this reason a motorcycle tire has a transverse curvature which is much greater than that of a motor vehicle tire; in the case of the former the transverse curvature coefficient is usually greater than 0.15, whereas in the second case it is less than 0.05.
It is known moreover that, in order to obtain good stability and good manoeuvrability of the vehicle, the front tire must have a smaller cross-sectional width, and hence a greater transverse curvature of the tread, preferably greater than the transverse curvature of the corresponding rear tire and, even more preferably, not less than 0.30.
The different disposition of the two types of vehicles has a considerably different effect on the behaviour of the respective tires and their structure, in particular along curvilinear trajectories, and results in deformations of the carcass which are not comparable with one another.
The marked behavioural difference between motor vehicle tires and motorcycle tires may be better understood by considering that, in brief, the former operate with a slip angle of about 30.degree. and a camber angle of 2.degree. or 3.degree., and the latter with a slip angle of 2.degree. or 3.degree. and a camber angle on average of 45.degree., with values increasing up to 65.degree. and more.
Therefore it was not foreseeable that a carcass fabric with low-count cords could produce improvements in motorcycle tires.
By way of conclusion, the improvements which have emerged from the comparative tests are due to the entirely random use of a fabric with low-count cords, available in the warehouse for tests on motor vehicle tires, and not a systematic selection of fabrics expressly intended to identify the best fabric for solving the problem.